Apps · Drive, safety and recovery
What are the "stand-by copies" Storage Studio cleared after an app move?
Short answer: During a move the original app is kept aside as a safety stand-by. After the move is finished and checked, the stand-by is cleared and its space is freed.
While an app is being moved, Storage Studio keeps the original set aside as a stand-by, so it can put it straight back if anything goes wrong. Once the move has finished and been checked, that stand-by is no longer needed and is cleared — which is when you get the space back.
The message simply reports how much space those finished stand-bys freed.